The present invention relates generally to the field of fiber optic cables. In particular, the present invention is directed to a novel foam-jacketed central strength member (also referred to as a cellular upjacketed rigid strength member) for fiber optic cables which demonstrates superior resistance to strain caused by thermal contraction or expansion.
Optical fibers are very small diameter glass strands which are capable of transmitting an optical signal over great distances, at very high speeds, and with relatively low signal loss as compared to standard wire or cable networks. The use of optical fibers in today's technology has developed into many widespread areas, such as: medicine, aviation, communications, etc. Because of this development, there is a growing need to have fiber optic cables operating with high efficiency with as little signal loss as possible.
At the center of common fiber optic cable is a central strength member. The central strength member has conventionally been made from a number of different materials, such as hard plastic, steel, glass, or a glass reinforced composite. The central strength member is used to bear cable installation loads so that fibers do not see strain during cable installation. The central strength member is the primary anti-buckling element in the cable; the central strength member resists cable contraction at low temperatures and prevents optical fiber buckling which would otherwise occur due to coefficient of expansion differential between optical fibers and other plastic cable components. In addition, the central strength member maintains buffer tube geometry; acts as a response member to compressive forces and provides a primary clamping point for hardware used to connect the cable to splice and routing enclosures.
Optical fibers can transmit more data more rapidly than copper wires; however, the use of optical fibers is not without its problems. One of the most important concerns when working with optical fibers is their sensitivity to damage during manufacture and installation and their sensitivity to bending and buckling. Great measures and developments have been made in attempts to protect fibers from damage during manufacture, installation, and use. Optical fiber performance is very sensitive to bending, buckling, or crushing stresses. Excessive stresses during manufacture, cable installation, or service can adversely affect the mechanical and optical performance of optical fibers.
When fibers are exposed to bending, buckling, or crushing stresses increased attenuation of the transmitted signals can result. If this increase in attenuation is high enough, the optical network in which the fiber is used can fail. Often, cables with fiber damage or cables without appropriate resistance to thermal contraction cannot be detected until a network goes into service. If a network fails due to damaged fiber or cables without an appropriate resistance to thermal contraction, high costs result from loss of service for customers and eventual replacement of defective cables.
Therefore, there is a need for providing fiber optic cable with sufficient structural properties to avoid damage and to better resist thermal contraction. The cable must have adequate tensile strength, resistance to crushing, resistance to buckling, and resistance to thermal contraction. These structural properties are frequently provided by rigid strength members.
At low temperatures, the polymeric materials that comprise most of the cable contract substantially more than the glass optical fibers. This differential strain may cause fiber strain or buckling. Such strain and buckling induced in an optical fiber will result in attenuation and possible network system failure. To mitigate this potential problem with differential cable and fiber strain, rigid elements are added to a cable as anti-buckling and anti-contractile elements. Examples of these rigid strength elements are metallic elements, glass reinforced composite rods, aramid reinforced composite rods, or composite rods made of some other high modulus, low coefficient of expansion material such as carbon fiber. Rigid strength members may be jacketed with solid polyethylene to obtain the proper outer diameter of the strength member required for the number and size of buffer tubes to be included in the cable. However, If the rigid strength members are coated, or upjacketed, with a large amount of solid polyethylene, resistance to contraction will be compromised because polyethylene has a coefficient of expansion orders of magnitude higher than that of the central strength member, which is made of glass reinforced composite, aramid reinforced composite, or metallic elements. The total contribution to low temperature contraction is equal to the coefficient of thermal expansion of the material multiplied by the temperature differential, modulus, and cross-sectional area of the material. To minimize total contractive force, a material with a low coefficient of thermal expansion is desired.